Motor vehicles such as electric vehicles or hybrid vehicles have been widely used in recent years. Motor vehicles require that a battery with a large capacity be mounted thereon, because a battery with a larger capacity provides a longer cruising range. It is therefore proposed to mount a battery pack that stores a plurality of battery modules under the floor of an automobile. JP 2011-121483 A, for example, proposes a structure in which a battery pack including battery frames extending in the vehicle forward-rearward direction is mounted under a floor panel such that portions of the battery frames on the vehicle rear end side are fastened, via a floor cross member, to rear side members.